1. Field
The disclosure relates generally to a low dropout regulator (LDO) circuits and methods and, more particularly, to a low dropout circuit device having improved limitation of startup inrush current and a method thereof.
2. Description of the Related Art
Low dropout (LDO) regulators are a type of voltage regulators used in conjunction with semiconductor devices, integrated circuit (IC), battery chargers, and other applications. Low dropout regulators (LDO) can be used in digital, analog, and power applications to deliver a regulated supply voltage.
An example of a prior art, a low dropout (LDO) regulator is illustrated in FIG. 1. An LDO regulator consists of an error amplifier 1, pass transistor 2, and a feedback network 3. The LDO regulator can be defined using bipolar transistors, or metal oxide semiconductor field effect transistors (MOSFETs). For a MOSFET-based implementation, the pass transistor 2 is typically a p-channel MOSFET device. The pass transistor 2 has a MOSFET source connected to voltage VDD, and whose MOSFET drain connected to output voltage, VOUT, and whose MOSFET gate is connected to the output of error amplifier 1. The error amplifier 1 has a negative input defined as voltage reference input, VREF, and a positive input signal feedback voltage, VFB. The feedback network 3 is connected between the p-channel MOSFET output voltage VOUT, and ground reference VSS. The feedback network 3 can consist of a resistor divider network whose output is the feedback voltage, VFB.
As illustrated in FIG. 2, the start-up current for a low dropout (LDO) regulator is shown in an LDO mode of operation. In the LDO mode of operation, there is a inrush current that exceeds the operational mode of a low dropout (LDO) regulator. This large inrush current is not desirable for low dropout (LDO) applications.
As illustrated in FIG. 3, the start-up current for a low dropout (LDO) regulator is shown in a Bypass mode of operation. In the Bypass mode of operation, there is an even larger inrush current that exceeds the operational mode of a low dropout (LDO) regulator. This large inrush current is not desirable for low dropout (LDO) applications.
In low dropout (LDO) regulators, the startup overshoot control has been discussed by modification of the feedback network through an output voltage based feedback loop. As discussed in published U.S. Pat. No. 7,402,987 to Lopata, a resistor element in the feedback loop is replaced by a variable resistor.
In low dropout (LDO) regulators, the startup overshoot control has been discussed by introduction of a soft-start. As discussed in published U.S. Pat. No. 7,459,891 to Al-Shyoukh et al., a control unit provides a control signal to a controllable resistor element to decrease incrementally in value.
In low dropout (LDO) regulators, the startup overshoot control has been discussed by buffering an associated supply input decoupling capacitor. As discussed in published U.S. Pat. Application 2006/0145673 to Fogg et al., a selectively configured current path is chosen that has a high impedance for startup charging of the decoupling capacitor, and a low impedance for normal operations of the circuit.
In these prior art embodiments, the solution to improve the response of the low dropout (LDO) regulator utilized modification of the resistors contained within the feedback or changing the charging of a capacitor.